Process and device for monitoring the degree of wear of a compression worm conveying plasticized material

ABSTRACT

An apparatus and a method for controlling a degree of attrition or wear of a sealing screw conveying plasticized material are provided. The apparatus includes an opening which intersects the wall of a housing of the screw in the region of the screw threads or in the region immediately neighboring the screw threads. Another opening intersects the wall of the housing. The two openings are spaced apart axially from each other and the pressure sensors connected to both openings. The process involved conveys a plasticized material against a resistance, the resistance being a sealing screw bearing supported within the screw housing. The pressure of the conveyed material is measured at two locations spaced apart from each other in the direction of conveyance. The two locations are disposed on both sides of a resistance body towards which the sealing screw conveys the material. The difference between the two measured values is used to indicate the degree of wear of the screw or its housing, respectively.

TECHNICAL FIELD

The invention relates to a process for controlling the degree ofattrition of a sealing screw conveying a plasticized material, inparticular synthetic plastic material, against a resistance body, whichscrew is bearingly supported within a screw housing between two screwsections located axially spaced apart from each other, the pitch of thethreads of the sealing screw being directed opposite to that of the twoscrew sections, wherein the housings of the two screw sections areinterconnected by a connecting channel leading the material conveyed bythe two screw sections, a station for processing the material flowingwithin the connection channel, in particular a filter station, beingconnected to the connection channel, and wherein the resistance body isdisposed between the sealing screw and that place at which theconnection channel branches off the housing of that screw sectionthrough which the material flows at first. Further, the inventionrelates to an apparatus for performing such a process.

BACKGROUND OF RELATED ART AND SUMMARY OF THE INVENTION

All screws bearingly supported within a screw housing and - although toa less degree - their housings are subject to wear, even when providedwith best armoring, in particular screws conveying a material,containing abrasive components, for instance more or less soiled, mainlythermoplastic, synthetic plastic material of the recycling industry.With screws conveying against an extruder head, wear leads to thedisadvantage that the pressure in front of the extruder head requiredfor a perfect extrusion of the material cannot more reliably built upsince the material slides back in an uncontrollable manner through theperiphery gaps between screw and housing, which gaps occur due to thewear. Particularly undesirable is the wear with sealing screws which inthe initially described manner are interposed between screw sections.Within this, the sealing screw is bridged by the connection channelforming a by-pass of the sealing screw. A wear in the region of thesealing screw, what occurs overtime, can be observed by gradual soilingof the material supplied to further processing, in particular to anextruder head, so that also the final product, as a rule a granulateobtained at the extruder head, is soiled. This causes complaints of thefirms processing the final granulate. Soiled granulate has a less valueand, in addition thereto, the granulate production of weeks mustpossibly be subjected to a second thermic filter processing, this;however, has the disadvantage that the second heat treatment requiresunnecessary power consumption and that the molecular chains of thesynthetic plastic material are adversely affected.

In order to avoid these difficulties, the sealing screw must bedismounted in regular time intervals and the housing as well as thescrew must be checked in view of wear or attrition. However, this work -as experience shows - is hardly ever made, because it is time consumingand must be done by experienced people, because the wear in the regionof the sealing screw can occur on this screw itself and this not only inview of the outer diameter of the screw threads, but also in view of thecore diameter of the sealing screw, as well as on the cylindrical screwhousing.

The invention has at its object to provide a process by which thecontrol of the degree of wear of a sealing screw is possible without anyproblem, so that it can be seen at any time to what degree the wear ofthe screw or also its housing has been increased, and this also duringoperation. The invention solves this task by the features that thepressure of the material conveyed by the screw section that is at firstpassed by the material and the pressure of the material conveyed by thesealing screw are measured, in front of the resistance body,respectively, and that the difference of the two pressures is used forindicating the degree of attrition. In this connection, the inventionstarts from the perception that the pressure onto the material conveyedby the screw in direction of conveyance of the screw, therefore towardsthe resistance body, always increases as long as the screw worksunobjectionably. If, therefore, the pressure at that place ofmeasurement which is reached by the conveyed material at first, isdesignated by P₁ and the measured pressure at the other place isdesignated by P₂, then the function of the screw is unobjectionable ifP₂ is greater than p₁. If both pressures P₁ and P₂ equalize, this so tobe understood that the attrition has already been much increased, what,as a rule, means already a critical limit. If P₂ becomes smaller thanP₁, then the function of the screw is no longer sufficient. Whennormally operating, namely, the perfect sealing screw conveys always asmall portion of the material supplied via the connection channel to thesecond screw section back in direction towards the first screw section.If the function of the sealing screw is unobjectionable, the pressure ofthe material at that place of measurement which is located nearer to thesealing screw, is always greater than the pressure of the material atthe other place of measurement. If both pressures become equal, thismeans that the sealing screw works neutrally, that is, no syntheticplastic material melt is conveyed by the sealing screw towards the firstscrew section. If the melt pressure at the place of measurementneighboring the sealing screw is smaller than at the other place ofmeasurement, this means that, for example, the sealing section formed bythe sealing screw is passed by unfiltered melt in direction towards theextruder head or towards an other outlet opening. The sealing functionof the sealing screw, therefore, is then no more given to a sufficientdegree and unfinished, in particular unfiltered, portions of thematerial will occur in the final product because the sealing screw thenforms an undesired by-pass for the connection channel leading to thetreatment station of the conveyed material, in particular to the filter.

“Screw threads”in this connection are to be understood by the volutionsof the screw.

For comparison measurements of synthetic plastics material it is knownto measure the pressure of the material at two places spaced apart fromeach other in a tube section which follows a screw section and has areduced diameter. Since the diameter of this tube section is constant,the pressure at the location which is at first reached by the syntheticplastic material, is greater than at the other location, caused by thewall friction of the plastic material at the inner wall of the tubesection. Further, no one of the two measurements is made within theregion of the screw threads or the screw housing, and the measuringresults obtained are only used for determining the properties of thesynthetic plastic material and not for controlling the degree of wear ofa screw.

The inventive apparatus for performing the process is characterized bythe fact that two screw sections axially spaced apart from each otherare interconnected for common rotation by a sealing screw, that anopening intersects the wall of the housing of the screw section that isat first passed by the material in the region of its screw threads or inan area immediately neighboring thereto, and a further openingintersects the wall of this housing or of the housing of the sealingscrew, the two openings being spaced apart from each other, measured inaxial direction of the screw sections, wherein the resistance body isdisposed in the region of this spacing, and that to each one of the twoopenings a pressure sensor is connected. The indications of thesepressure sensors are already sufficient for controlling the degree ofwear of the screw or its housing in the above explained manner. However,it is more favorable, if transducers are connected to the pressuresensors, which at their output deliver an electrical signal to acomparison circuit, to the output of which an indication instrument isconnected. This comparison circuit controls continuously the pressuredifference of P₁ and P₂ and indicates it by means of the indicationinstrument. This instrument, in a manner known per se, can be soconstructed that it produces a warning signal as soon as the pressuredifference decreases a predetermined value. This warning signal may beof acoustic or visual kind and, if desired, may be immediately used forstopping the plant.

It is particularly favorable, if according to a further embodiment, atleast one of the two openings in the wall of the screw housing branchesoff an annular channel neighboring the screw threads of the screwsection or of the sealing screw. This has the disadvantage that themeasuring result or the pressure sensor connected to the respectiveopening is free of impulses.

According to a further embodiment, the resistance body may be formed bya baffle body which is disposed between the two openings and dams up thesupplied material and is disposed on a shaft connecting the screwsections with the sealing screw. This results in a simple constructioninasmuch this baffle body may have a plurality of constructions. Thesimpliest embodiment consists in that the baffle body is a disk whichconfines a narrow annular gap between its periphery and the wall of thehousing of the sealing screw; a further embodiment of this constructionmay consist in that at least one annular groove is provided on theperiphery of the disk. This annular groove causes turbulences in theplasticized material passing the disk, what increases the resistance forthis material. Alternatively, baffle body may be a screw thread section,the screw threads of which have the same conveyance direction as thescrew threads of the sealing screws, but are finer. These fine screwthreads cannot convey the entire material supplied by the comparativelycoarser screw threads of the sealing screw. The difference in the amountof the material conveyed by the sealing screw, therefore, is “shifted”over the fine screw threads, that means that it is pressed pass the finescrew threads, what causes an increase of pressure at the end of thesealing screw.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the invention is schematically shown in thedrawings by way of exemplary embodiments.

FIG. 1 shows a longitudinal section through a first embodiment having anapparatus provided with a sealing screw, and the course of the materialpressure within this apparatus is shown by a graphical representation.

FIG. 2 shows a first variant of this embodiment, also with acorresponding graphical representation of the material pressure courseand

FIG. 3 shows a further embodiment, also with the respective materialpressure course.

FIG. 4 shows a detail of FIG. 3 in an enlarged scale.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT

Within the apparatus according to FIG. 1, the material to be processed,in particular soiled thermoplastic synthetic plastics material, obtainedfrom waste products, if desired after a pre-comminuting process, isfilled into a hopper 1 in direction of the arrow 2 and thus reachesthrough an intake opening 3 the interior of a screw housing 4 in which ascrew 5 is bearingly supported. This screw 5 has two sections 6, 7axially spaced apart from each other, between which a sealing screw 8 isdisposed, the pitch of the screw threads thereof being oppositelydirected to the pitch of the screw threads of the two sections 6, 7. Thetwo screw sections 6, 7 and the sealing screw 8 are disposed on a commonshaft 9 that is so rotated by a motor 10 that the material introducedthrough the intake opening 3 is conveyed by the two screw sections 6, 7in direction of the arrow 11, for example towards an extruder head.Suitably, the two screw sections 6, 7 and the sealing screw 8 have equalouter diameter of the screw threads, what has the advantage that thehousing 4 bearingly supporting all of these screws can have acontinuously constant inner diameter. At the intake end of the screw 5,sealing screw threads 14 are provided which have a reduced pitch and areduced lead, when compared with the screw sections 6, 7.

For by-passing the sealing screw 8, a connection channel 13 is providedwhich intersects the housing sections associated to the screw sections 6and 7 at both sides of the sealing screw 8 and in which the materialconveyed by the screw sections 6, 7 flows in direction of the arrows 15.A station 16 for processing the material flowing in the connectionchannel 13. This material processing may be of different kind, forexample an addition of filling materials, colouring matter or otheradditions. In the most cases, if desired in addition to the abovedescribed possibilities, the station 16 is a filter station 17, thefilters thereof filtering the impurities, in particular solid bodies ofall kind, out of the plasticized synthetic plastic material, so thatcleaned synthetic plastic material from the connection channel 13 entersthe section of the housing 4 surrounding the screw section 7 and isconveyed by the screw section 7 towards the exit, in particular towardsan extruder head. As it can be seen, the clearness of the materialconveyed by the screw section 7 towards the extruder head, depends fromthe sealing function of the sealing screw 8. If this sealing function isinsufficient, for example due to wear of the sealing screw 8 and/or ofits housing, no unobjectionable sealing between the two screw sections6, 7 is given more, so that soiled material conveyed by the screwsection 6 reaches the screw section 7 without having passed theconnection channel 13 and the filter inserted into it. This means, thatunfiltered, therefore soiled, material is conveyed by the screw section7 towards the extruder head or towards the other outlet and ultimatelycan be noticed by a pollution of the produced final product, as a rulegranulate. If the station 16 is not or not exclusively a filter station17, but for example adds additives, then the desired mixing ratio of thematerial supplied to the extruder head or the like cannot be reliablykept more, but depends from the sealing function of the sealing screw 8.

In order to continuously control the sealing function of the sealingscrew 8, two openings 18, 19 are provided which intersect the wall 20 ofthe screw housing 4 and are disposed spaced apart from each other, whenseen in axial direction of the housing 4. The one opening 18 is locatedin the region of the end of the screw threads of the screw section 6 orimmediately neighbouring this end, the other opening 19 is displacedwith respect to the opening 18 in direction of the arrow 11 and islocated in a region of the housing 4 adjacent to the screw threads ofthe screw section 6, however still in front of the sealing screw 8, whenseen in direction of the arrow 11. To each one of the two openings 18,19 a pressure sensor 22 or 23, respectively, is connected by means ofpressure lines 21. By means of these two pressure sensors 22, 23,therefore, the pressure of the material conveyed by the apparatus can bemeasured at the respective location. Between the two openings 18, 19there is disposed a resistance body 12 put onto the shaft 9, which bodywithin this embodiment is formed by a disk-shaped baffle body 26,between the periphery thereof and the inner wall of the housing sectionsurrounding the resistance body 12, there being a narrow annular gap 35through which the material conveyed by the sealing section 8 can enterthe housing section of the screw section 6. This damming body 26 has nothreads and acts so that it builts up an increased resistance for thematerial conveyed by the sealing screw 8, which resistance must beovercome by the sealing screw 8.

Therefore, a perfect sealing screw in normal operation conveys always asmall portion of the filtered material back (against direction of thearrow 11) into the housing of the screw section 6. The resistance body12 causes that the material pressure P₂ sensed by the pressure sensor 23is always greater than the material pressure P₁ sensed by the pressuresensor 22, if the sealing screw 8 has a perfect function. The differencevalue P₂−P₁ therefore, is a value from which conclusions can be drawnfor the sealing condition of the sealing screw 8. If the two pressuresP₂ and P₁ become equal, this means that the sealing screw worksneutrally, that means, no synthetic plastic melt is conveyed by thesealing screw 8 against the direction of the arrow 11. If P₂ becomessmaller than P₁ this means that the sealing section formed by thesealing screw 8 is passed in extrusion direction (arrow 11) byunfiltered synthetic plastic melt. Then, the sealing function is no moreensured and the final product is soiled in an undesirable manner.

FIG. 1 shows also the course of pressure p in dependence from the lengthof path s measured in axial direction of the screw 5 in form of adiagram. By full lines, the pressure course for an ideal condition ofthe sealing screw is shown, by broken lines a condition is shown atwhich the sealing screw is “passed” by unfiltered, plasticized syntheticplastic material in direction of the arrow 11. It can be seen that for aperfect sealing screw the conveyance action of the sealing screw 8against the resistance body 12 produces a pressure value at theupstream-side end of the sealing screw 8, therefore at the opening 19,which pressure is greater than the pressure at all other locations ofthe plant, in particular at those locations of the apparatus, which arelocated in front of the opening 19, when seen in direction of the arrow11. If, however, the sealing screw 8 or the housing section surroundingit shows unduly great attrition or wear, the pressure at the opening 19decreases so, that this pressure is no more greater than the pressure atthe opening 18, but smaller.

Suitably, an annular channel 24 or 25, respectively, each, is disposedin the region of the two openings 18, 19, which channels are free fromscrew threads, in order to make the measuring results of the twopressure sensors 22, 23 independent from pulsations introduced via thescrew threads.

Transducers 31, 32 (FIG. 1) may be connected to both pressure sensors22, 23, which transducers transform the pressure values measured by thepressure sensors 22, 23 into electrical values. The output of the twotransducers 31, 32 is connected to a comparison circuit 33, which in amanner known per se compares the electrical signals delivered by thetransducers 31, 32 and indicates the difference of the two signal valuesat an indication device 34 in an optical and/or acoustical manner. Thesignal obtained by the comparison circuit 33 may also be used forautomatically stopping the apparatus if a degree of attrition is noticedwhich exceeds a predetermined value.

The embodiment according to FIG. 2 differs from that according to FIG. 1only in that two annular grooves 36 are provided on the periphery of theresistance body 12 formed by a disk-shaped baffle body 26, which groovesintroduce turbulences into the material flowing within the gap 35 sothat the resistance for the flow within the gap 35 is increased. Besidesof this, the apparatus according to FIG. 2 corresponds to that accordingto FIG. 1, the members connected to the two pressure sensors 22, 23being not shown in FIG. 2 for simplification's sake, and also not thestation 16.

Also FIG. 2 shows the course of pressure in the apparatus by way of adiagram, and again there indicates a full line the pressure course for aperfect sealing screw 8, whereas the interrupted lines indicate theconditions for an imperfect function of the sealing screw 8.

The embodiment according to FIGS. 3 and 4 differs from those accordingto FIGS. 1 and 2 in that the baffle body 26 on its periphery is providedwith fine threads 37. These fine threads 37 indeed have the same pitchdirection as the threads or the volutions of the sealing screw 8,however, the fine threads are much finer than the latter threads. Thefine threads 37, therefore, cannot convey the entire material suppliedby the sealing screw 8 against the direction of the arrow 11 indirection towards the screw thread 6. A portion of this material,therefore, is conveyed through the annular gap 35 along the volutions 38(FIG. 4) of the fine threads 37 towards the left (FIG. 3), so that—forexample the fine threads 37 are “passed”. Thereby a resistance for thematerial conveyed by the sealing screw 8 is created, so that—if thesealing screw 8 works perfectly—there is the pressure course shown inthe diagram by full lines, and this pressure course changes to thatshown by interrupted lines if the sealing screw 8 and/or its housingsection show a higher wear.

What is claimed is:
 1. A process for controlling the degree of attritionof a sealing screw conveying a plasticized material against a resistancebody, which sealing screw is bearingly supported within a screw housingbetween a first screw section and a second screw section disposedaxially spaced apart from each other, the pitch of the threads of thesealing screw being directed opposite to the pitch of the first andsecond screw sections wherein a first housing section of the first screwsection and a second housing section of the second screw section areinterconnected by a connection channel leading the material conveyed bythe first and second screw sections, a station for processing thematerial flowing within this connection channel being connected to theconnection channel, and wherein the resistance body is disposed betweenthe sealing screw and that place at which the connection channelbranches off the first housing section of the first screw sectionthrough which the material flows at first, wherein the pressure of thematerial conveyed by the first screw section and the pressure of thematerial conveyed by the sealing screw are measured in front of theresistance body and whereby the difference of the two pressures is usedfor indicating the degree of attrition.
 2. An apparatus for controllingthe degree of attrition of a sealing screw conveying a plasticizedmaterial, comprising: a screw housing having a wall; a sealing screwdisposed within the screw housing; an upstream screw section having anupstream end and a downstream end and disposed within an upstreamhousing section of the screw housing, a downstream screw sectiondisposed within a downstream section of the screw housing, the upstreamscrew section and the downstream screw section being axially spacedapart and interconnected for common rotation by the sealing screw; aconnecting channel bypassing the sealing screw, the connecting channelincluding a first opening intersecting the housing wall of the upstreamhousing section proximate the downstream end of the upstream screwsection and a second opening intersecting the housing wall proximate thesealing screw; and a station for processing the material flowing withinthe connection channel fluidly connected to the connection channel;wherein the first and the second openings are axially spaced apart, andfurther including a resistance body disposed between the upstream screwsection and the scaling screw, a third opening intersecting the housingwall of the upstream housing section near the downstream end thereof, afourth opening intersecting the housing wall of the screw housing of thesealing screw in front of its end facing the upstream screw section, afirst pressure sensor connected to third opening, and a second pressuresensor connected to the fourth opening.
 3. The apparatus of claim 2,wherein transducers are connected to the pressure sensors, whichtransducers at their outputs deliver electrical signals to a comparisoncircuit, to the output of which an indication device is connected. 4.The apparatus of claim 2, wherein at least one of the first and thirdopenings in the housing wall branches off an annular channel neighboringthe screw threads of the upstream screw section.
 5. The apparatus ofclaim 2, wherein at least one of the first and third openings in thehousing wall branches off an annular channel neighboring the screwthreads of the sealing screw.
 6. The apparatus of claim 2, wherein theresistance body is formed by a baffle body, said baffle body beingprovided on a shaft connecting the screw sections with the sealingscrew.
 7. The apparatus of claim 6, wherein the baffle body is a disk, anarrow annular gap being provided between the periphery of the disk andthe wall of the housing section surrounding the sealing screw.
 8. Theapparatus of claim 7, wherein at least one annular groove is provided onthe periphery of the disk.
 9. The apparatus of claim 6, wherein thebaffle body includes a screw thread section having screw threads finerthan the screw threads of the sealing screw, and the screw threads ofthe screw thread section having the same conveyance direction as thescrew threads of the sealing screw.
 10. The apparatus of claim 2,wherein the two screw sections have the same outer diameter as thediameter of the sealing screw threads.
 11. The apparatus of claim 2,wherein the sealing screw has an outer diameter of its screw threadswhich is equal to the outer diameter of the screw threads of at leastone of the two screw sections.